Intraspecific variation within gypsum endemic species of Nama (Boraginales).

Exposed patches of gypsum are distributed in an island-like fashion throughout the Chihuahuan Desert region, and host a diverse endemic flora of more than 200 taxa in over 30 plant families. Gene flow within gypsum endemic taxa shouldbe limited among distant gypsum patches, which should promote genetic differentiation among populations restricted to gypsum and favor allopatric speciation, given sufficient time. In support of this idea, a number of plant genera in the Chihuahuan Desert have several allopatric gypsum endemic taxa within them, including Nama (Boraginales), a genus of ~60 species that includes 10 gypsum endemic taxa. A phylogenetic study of Nama by Taylor and Simpson (2008) revealed that gypsum endemism has evolved multiple times within the genus,including at the base of a large clade of 10 species, 8 of which are restricted to gypsum. However, this study was unable to address diversity within taxa due to limited sampling. We sampled multiple populations of all gypsum endemic species of Nama from across their geographic ranges to determine whether genetic diversity within these speciesis correlated with geography, as might be expected given their wide distribution. To address this question we sequenced ITS and the plastid ndhF/rpl32 and rpl32/trnL spacer regions for all individuals. These data confirm that gypsum endemism has evolved multiple times in Nama. Within the large gypsum endemic clade, both ITS and plastid data strongly support the individual monophyly of the species N. carnosum, N. hitchcockii, and N. canescens, and also recover a genetically diverse clade of 5 species that are centered on Coahuila. Although no molecular dating analyses were pursued, the relatively long branch lengths in the gypsum endemic clade,combined with its morphological diversity and the fact that several species demonstrate haplotype coalescence in both plastid and ITS data, suggest that this clade, and thus gypsum endemism, may be several million years old.